The use of blowout preventers in drilling, completion, workover, and production of oil and gas wells is well known. Such blowout preventers generally include a housing with a bore extending through the housing. Opposed chambers extend laterally on either side of the bore in the housing and communicate with the bore. Rams within the chambers are connected to rods that are supported for moving the rams inwardly into the bore to close off the bore. This action divides the bore into a zone above the rams and a zone below the rams. The rods also serve to retract outwardly from the bore to open the bore.
Various types of rams may be employed such as those which engage circumferentially around a pipe or tubular member for sealing engagement with the tube or pipe, while others are provided with cutting surfaces for shearing tubular members or cables which extend through the bore of the blowout preventer.
Blowout preventers (BOPs) are also commonly used in coiled tubing systems. Such BOPs provide a means of holding the tubing and isolating the well bore pressure during a variety of conditions, including emergencies. The configuration of the BOP rams and sideport facility allows well-control operations to be conducted under a variety of conditions.
Newer blowout preventers include four sets of rams, which may be referred to herein as a “Quad BOP”. The system comprises a set of four stacked elements, each with a different function. Blind rams are shut when there is no tubing or tool string extending through the body of the BOP. Shear rams are designed to close on and cut through the tubing. Slip rams close on and hold the tubing, ideally without damaging the surface of the piping or other tubular member. Finally, pipe rams seal around the tubing when it is in place. Each of the rams should only be actuated when the tubing is stationary; otherwise, damage to either the BOP or the tubing is likely. Of the four types of rams just described, the present invention is directed to the slip ram type for use with tubing.
As previously explained, a slip ram closes onto a tubular, and in the case of the present invention, closes on and holds tubing. Slip segments to grip and suspend coiled tubing are well known and widely used in coiled tubing applications. The slips are typically installed in a set of rams. The slips are most often made in two pieces, one piece in each ram, with gripper teeth on the semi circle resulting in near 360 degrees coverage of the coiled tubing diameter. The gripper section is machined to a specific inside diameter to match the outside diameter of the coiled tubing. This system works reasonably well as long as the coiled tubing is of a constant diameter. Over-worked coiled tubing may become undersized, oversized, or out of round, all of which reduce, or negate the effectiveness of the slip segment gripper teeth.
Furthermore, recent innovations have provided tubing which has a substantially constant inside diameter, but a substantially constantly increasing outside diameter, so that the tubing presents a tapered aspect in its outside diameter. Development of such a tapered outside diameter coiled tubing renders the gripping system with a set diameter unworkable. In other words, with a first length of tubing through the slip ram, a relatively small diameter of tubing must be accommodated by the slip ram. However, with a longer length of tubing down hole, a larger diameter of tubing must be grasped and held. Current structures of slip rams offer a set diameter of the ram, provided in equal halves on either side of the tubular, and this is incapable of accommodating the varying diameter of tubing which is presented to the slip ram, if the outside diameter of the tubing varies with length. It is believed that the prior art has failed to solve, or even address this problem.
In summary, as coiled tubing technology has advanced, the need to go deeper has also advanced. Inherent problems with increased depth are many, included among these is increased tubing string weight. One method of reducing string weight is to use different sizes of coiled tubing joined together. Therefore the need arises to be able to perform all of the conventional pressure control methods, one of which, and the subject of this invention, is to grip and hold the variable size tubing, including the transition zone. The present invention addresses this need in the art.